1. Field of the Invention
The present invention relates to a manufacturing method for a wear resistant sintered member which is advantageously used for various sliding members. In particular, the present invention relates to a manufacturing method for a wear resistant sintered member which slides at high pressures. In addition, the present invention relates to a sintered valve seat for automobile engines, and relates to a production method therefor. In particular, the present invention relates to development techniques of sintered valve seat which is advantageously used for high load engines (for example, compression natural gas (=CNG) engines and heavy duty diesel engines).
2. Description of the Related Art
In sintered components manufactured by a powder metallurgy method, a desired hard phase is easily dispersed into an alloy matrix, and the sintered components are used for various sliding members (for example, sliding members for internal combustion engines and bearings used for automobiles and the like). However, in recent years, practical environments are severer in accordance with high performance of apparatuses having a sliding member provided therein. In order to cope with it, high wear resistance is required in sintered sliding members. In addition, wear resistance in various environments are required to be improved in accordance with wide application of sintered sliding members. For example, wear resistance and oxidation resistance at high temperatures are required depending on portions at which the sintered sliding members are used.
In order to meet the above requirements, Japanese Unexamined Patent Application Publications Nos. H08-109450, H02-270943, and H01-068447 (hereinafter referred to as “Patent Publication 1”, “Patent Publication 2” and “Patent Publication 3”) propose a wear resistant sintered member for various applications, in which a Co—Mo—Si—Cr based hard phase or a high-speed steel based hard phase which is dispersed. Japanese Unexamined Patent Application Publication Nos. 2002-356704, 2003-119542, and H01-154798 (hereinafter referred to as “Patent Publication 4”, “Patent Publication 5” and “Patent Publication 6”) propose a wear resistant sintered member in which various improved hard phase are dispersed.
In particular, in recent years, operation conditions of automobile engine are very sever due to high performance thereof. Valve seats for engine are required to endure practical environments which are severer than those of conventional techniques. For example, in liquefied petroleum gas (=LPG) engine widely used in automobile for taxi, since sliding surfaces of valve and valve seat are used in dry conditions, wear occurs more quickly than that in valve seat for gasoline engine. In environments in which sludge is adhered to a valve seat for highly leaded gasoline engine, when a pressure to valve seat is high or when a valve seat is used in high temperature and high compression ratio condition, wear is accelerated by the sludge. When a valve seat is used in the above severe environments, the valve seat is required to have good wear resistance and high strength for prevention of deformation by plastic flow in high temperature.
On the other hand, a valve operating mechanism equipped with a rush adjuster, which can automatically control a position of valve and a drive timing of valve even when valve seat is worn, has been used. However, an engine life problem due to wear of a valve seat is not solved, and development of materials for a valve seat which is superior in wear resistance is desired. In recent years, not only high performance but also development of inexpensive automobile has been important from viewpoint of economic efficiency. Therefore, a sintered alloy for a valve seat is required to be highly wear resistant and have a high strength without an additional mechanism such as the above rush adjuster.
Regarding the above sintered alloy used for a valve seat, Japanese Examined Patent Application Publication No. S59-037343 (hereinafter referred to as “Patent Publication 7”) proposes a technique in which Co—Mo—Si based hard particles are dispersed into a dapped matrix of Fe—Co based matrix and an Fe—Cr based matrix. Japanese Examined Patent Application Publication No. H05-055593 (hereinafter referred to as “Patent Publication 8”) proposes a technique in which Co—Mo—Si based hard particles are dispersed into an Fe—Co based matrix. Japanese Examined Patent Application Publication No. H07-098985 (hereinafter referred to as “Patent Publication 9”) proposes a technique in which Co—Mo—Si based hard particles are dispersed into an Fe—Co based matrix containing Ni. Japanese Unexamined Patent Application Publication No. H02-163351 hereinafter referred to as “Patent Publication 10”) proposes an Fe base alloy including Co—Mo—Si base hard particles dispersed thereinto.